Different frequency bands are used for different radio communication services, such as mobile communication, small power data communication, radio frequency identification (RFID), etc. For example, so-called third generation mobile communication systems use frequency bands from 810 to 958 MHz, 1428 to 1525 MHz, 1750 to 1785 MHz, 1845 to 1880 MHz, 2110 to 2170 MHz, etc. On the other hand, the global positioning system (GPS) uses a frequency band from 1563 to 1578 MHz. For local area networks (LANs), frequency bands from 2.4 to 2.5 GHz and 5.47 to 5.725 GHz are used.
In recent years, communication devices, such as mobile phones, have come to be designed to support a plurality of such radio communication services, such as described above, in order to enhance user convenience. Each such communication device is mounted with different antennas for different frequency bands in order to transmit and receive radio signals at different frequency bands used for different radio communication services. However, from the standpoint of reducing the size of the communication device, it is desirable to reduce the number of antennas mounted in the communication device.
In view of the above, research has been carried out to develop an antenna having good antenna characteristics over a wide range of radio signal frequencies (for example, refer to Japanese Laid-open Patent Publication No. 2004-96341, International Publication WO2007/094111, Japanese Laid-open Patent Publication No. 2005-64596, and “Design of Ultrawideband Mobile Phone Stubby Antenna (824 MHz-6 GHz)” by Zhijun Zhang and three others, IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, IEE, July 2008, Vol. 56, No. 7, pp. 2107-2111).
In one prior art example, an antenna having a three-dimensional shape is used which is fabricated by folding a V-shaped stamped sheet metal. A capacitor and an inductor are connected in series to the antenna to which are also connected an inductor and a capacitor for short-circuiting the antenna to ground. This antenna has good antenna characteristics for radio frequencies ranging, for example, from 0.8 GHz to 10.6 GHz.
In another prior art example, resonant frequency is adjusted by selectively coupling one of a plurality of inverted-F antennas to a feed line via a switch. In this prior art example, each inverted-F antenna includes at least two antenna conductive elements coupled in series via a switch. Then, the resonant frequency is adjusted by controlling the switch so as to vary the effective length of the antenna.
In still another prior art example, the resonant frequency of a feeder/radiating electrode is adjusted by turning on or off the conduction of a conduction path electrically connecting between a capacitive loading means for loading a capacitance on a higher order mode zero voltage region of the feeder/radiating electrode and a ground electrode. The antenna structure according to this prior art example has good antenna characteristics for radio frequencies ranging, for example, from 0.7 GHz to 2.3 GHz.
In yet another prior art example, the length or thickness of a ground terminal and a feeder terminal connected to a conductor formed as a radiation pattern is varied in order to adjust the antenna impedance.
However, in the Long Term Evolution (LTE), a mobile communication standard for which work on standardization is proceeding in the Third Generation Partnership Project (3GPP), it is expected to also use the 0.7-GHz band. Further, as earlier noted, in wireless LANs, the frequency band from 5.47 to 5.725 GHz is used. There is therefore a need for an antenna device having good antenna characteristics over a wider range of radio frequencies, for example, radio frequencies ranging from 0.7 GHz to 6 GHz.